1. Field of the Invention
The present invention relates to a method of recovering a palladium catalyst. More particularly, the invention relates to a method of recovering a palladium component from a liquid reaction mixture which has been produced by oxidative coupling reaction of an aromatic compound in the presence of the catalyst containing palladium(II) salt.
2. Description of Prior Art
A catalyst containing palladium(II) salt is very useful as a catalyst employable in organic synthesis, for example, that for obtaining a dimer such as tetramethyl biphenyltetracarboxylate from an aromatic compound (e.g., dimethyl phthalate) through oxidative coupling reaction. Since the catalyst is expensive, it is generally used repeatedly after recovery in the form of a metallic palladium (also referred to as "palladium black") after the reaction is complete.
For recovering palladium black, generally known is a method comprising the steps of heating a liquid reaction mixture at a temperature of approx. 60.degree. to 90.degree. C. in a hydrogen gas-containing atmosphere in an autoclave after the above-mentioned reaction is complete, to perform a reduction reaction and to precipitate palladium black, or reducing the liquid reaction mixture at room temperature or a temperture of approx. 60.degree. to 70.degree. C. using a reducing agent such as sodium borohydride or sodium formate to precipitate palladium black, and separating the palladium black from the liquid reaction mixture by filtration, as described in Japanese Patent Publication No. 53(1978)-20009.
However, the above-mentioned method has various problems. For example, the precipitate (i.e., deposit) such as palladium black produced by the reduction reaction in the above-mentioned method is dispersed in the liquid reaction mixture to form a homogeneous slurry with the liquid, and comprises both extremely small sized particles and relatively large sized particles whose diameters are distributed widely in the range of from 0.1 to 100 .mu.m. Particularly, the extremely small sized particles having a diameter of less than 5 .mu.m occupy approx. 20 wt. % of all the particles, and such small sized particles of palladium black hardly sedimented even after the liquid reaction mixture is allowed to stand for a long period of time or hardly separated from the liquid reaction mixture even by filtration. Hence, the recovery yield (i.e., recovery ratio) of palladium black is not high.
Further, in the case that the extremely small sized particles of palladium black are not recovered and still remain in the liquid reaction mixture, they bring about adverse effects such as decomposition of a dimerization product (e.g., tetramethyl biphenyltetracarboxylate) and modification thereof when the dimerization product is subjected to a separation procedure from the liquid reaction mixture or a purifying procedure.
In view of the above-described problems, there has been made a proposal to incorporate an alkali metal carbonate such as sodium carbonate or sodium bicarbonate into the reaction mixture when the catalyst in liquid reaction mixture is reduced into metallic palladium, as described in Japanese Patent Publication No. 60(1985)-59974. By adopting this method, relatively large sized particles of palladium black are liable to precipitate, so that a high recovery yield of palladium black can be obtained and deterioration of reaction product can be prevented.
However, the above-mentioned improved method is sometimes industrially disadvantageous, because the method needs an additional procedure of incorporating an alkali metal carbonate.